Magnets that find it hard to relax

Many processes in nature, such as alpha decay of U-238, or the conversion of ortho to para hydrogen, take place very slowly, and in each case, an investigation into the causes teaches us important physics. In this talk, we will discuss magnetic relaxation in single molecule magnets such as Fe8, which is also very slow, but for completely different reasons than the previous two examples.

At low temperatures, Fe8 displays spectacular quantum dynamics wherein the spin angular momentum degree of freedom of one molecule tunnels through 20 units of hbar, with a tunnel splitting of ~ 1 pico eV, or about 1kHz in frequency units. This splitting is about 106 times smaller than the energy bias on a typical molecule in Fe8 crystals due to the dipole-dipole interaction. As a result, tunneling is strongly inhibited by energy conservation, and essentially impossible. The overall relaxation of magnetization is thus exceptionally slow, and understanding it is a challenging problem in classical many-body physics.

We will describe the theoretical model for how we believe Fe8 relaxes, along with Monte Carlo simulations and kinetic equations for the spin and dipolar-field distributions. We apply these approaches to various experimental protocols. The agreement between simulations, kinetic equation, and experiments is very good in most respects, but not so good for ultra long times and ultra-slow phenomena. The problem of how an initially demagnetized sample can be magnetized is particularly interesting and still open, and entails a situation where the magnetization relaxes.

Host: Mark Meisel